The present parper discusses the initiation and growth of trees in polystrene and polyvinyl chrolide for the range of temperature from 20°C to 80°C.
The relation between the applied voltage (
V) and the time (
t) for tree initiation at 50% of full growth is given by
log
t=Const-
nlog
V (1)
At each temperature there is a critical applied voltage (
Vc) beyond which the value of n begins to decrease. From the values of n obtained by eq. (1) it is considered that above
Vc, where the voltage dependence is large, the trees initiate by electrothermal breakdown. On the other hand, below
Vc, where the voltage dependence is small, the trees appear to initiate by electromechanical breakdown.
For polymeric dielectric materials (for example, silicone rubber) whose Young's modulus is below 10
8 dyn/cm
2, a void is formed between the needle tip and polymer. The void is thought to be formed by Maxwell stress.
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